This invention is concerned with improvements in or relating to a process for reducing water flow from a hydrocarbon production well and is particularly, although not exclusively, concerned with a process for forming a water shut-off in a gas well.
Known gas well water shut-off processes include the UNOCAL alternating polymer-nitrogen injection process which has been used with a degree of success. The UNOCAL process is described in Society of Petroleum Engineers (SPE) 26653, xe2x80x9cNew Polymer Technology for Water Control in Gas Wellsxe2x80x9d, H. T. Dovan and R. D. Hutchins, Unocal Energy Resources Technology (SPE Production and Facilities, November 1994). The UNOCAL process uses a cross-linking polymer to produce a gel block to water production with nitrogen injection forming channels for gas flow. Testing of the UNOCAL process under laboratory conditions raised doubts regarding the reliability and durability of the process. It is also known to use relative permeability modifiers, i.e. chemicals that reduce the reservoir permeability to water with minimum effect on hydrocarbon permeability and while these relative permeability modifiers represent a low risk in terms of impairment of gas production they are not wholly effective as water shut-off means in addition to which their durability is somewhat limited.
To overcome the perceived difficulty in the known processes, a permanent water shut-off mechanism was established which uses a durable polymer system. It was therefore realized that there could be unforeseen advantages over prior art processes such as the UNOCAL process or processes using the relative permeability modifiers, in combining the attributes of the UNOCAL system with the reliable MARASEAL gel system and using the combination of processes to selectively place a reliable and durable water shut-off in a water zone of a gas well. It is, therefore, an object of the present invention to mitigate or overcome the shortcomings of the prior art processes referred to herein.
Thus, the present invention conveniently provides a process for reducing water flow from a hydrocarbon production well comprising the steps of:
(a) injecting a first chemical composition reactable to form a first polymer effective as a relative permeability modifier polymer into the hydrocarbon and water zones of a reservoir;
(b) injecting a second chemical composition into said hydrocarbon and water zones, the second chemical composition reactable to form a flow blocking polymer;
(c) effecting a xe2x80x9cshut-inxe2x80x9d for a reaction period of the first chemical composition to form the relative permeability modifier polymer;
(d) producing a second polymer composition from said second chemical composition out from the hydrocarbon zones prior to reaction of the second polymer composition; and
(e) maintaining the production of the first and second polymer compositions from the hydrocarbon zones to facilitate the creation of a passage for hydrocarbon flow therefrom while enabling the second polymer composition to react in the water zones to form a water shut-off.
Preferably step (b) of the process is effected immediately upon the completion of step (a) thereof. Advantageously, the first polymer composition comprises a cationic polyacrylamide and the second polymer composition comprises a cross-linkable polyacrylamide-chromium acetate system. Preferably, any reaction period for the first polymer composition with a cross-linking agent is less than the delay period prior to reaction of the second polymer composition.
The present invention also provides a process for forming a water shut-off including the injection of a relative permeability modifying polymer composition and a flow blocking polymer composition into hydrocarbon and water zones of a reservoir. The process comprises the production of the polymer compositions from the hydrocarbon zones concommitantly with the reaction of the flow blocking polymer composition in the water zones whereby a water shut-off is formed in the water zones. Conveniently, the production of the polymer compositions from the hydrocarbon zones if facilitated by reservoir pressure.